What factors affect how sediments containing nutrients like phosphorus enter Lake Erie, ultimately forming harmful algal blooms?
Researchers lack crucial information about the time sediments spend moving through stream networks (i.e., their “recency”), the sources of the sediments, and how these factors influence the exchange of phosphorus between sediment and its dissolved form in water. These dynamics could influence the magnitude, longevity, and bioavailability of phosphorus entering Lake Erie and the formation of algal blooms.
Morgan Shaw, graduate student at The Ohio State University’s Aquatic Ecology Laboratory, ran experiments using sediment collected from both riverbanks and agricultural fields.
To address these knowledge gaps, researchers led by Dr. Jim Hood of The Ohio State University studied how sediment composition, source, and age influence phosphorus cycling in streams. The project was funded through the Ohio Department of Higher Education’s Harmful Algal Bloom Research Initiative (ODHE HABRI).
“What we were interested in understanding is: where is that sediment coming from? How long is it staying in the river?” said Hood, an associate professor within the Department of Evolution, Ecology and Organismal Biology. “How do those two things influence how sediment interacts with phosphorus as it moves downstream?”
At different sites in the Black Creek watershed in northwest Ohio, researchers took measurements and analyzed samples during low flow in the summer and during high flow events associated with storms. The team also conducted a short-term, realistic mesocosm experiment to study how sediment source influenced phosphorus exchange between dissolved and particulate forms.
While still preliminary, results from the project have helped reduce uncertainty about phosphorus exports into Lake Erie. Results suggest that sediment varies widely in how long it stays in a river system, from less than 5 months to a decade. This information will be useful for targeting agricultural best management practices and modeling time lags between phosphorus losses from land and exports into Lake Erie.
The team collected significant data about how sediment characteristics shape phosphorus sorption rates and how these rates vary across the landscape. During low flow conditions, riverbed sediment samples were predicted to sorb phosphorus, serving as a phosphorus “sink,” and this potential was lower at downstream sites where dissolved reactive phosphorus was lower. During high flow events, suspended sediment was also predicted to be a phosphorus sink.
Researchers collected sediment samples from the Black Creek watershed, hoping to better understand how sediment moves through stream networks.
In the team’s experiments, sediment derived from cropland didn’t sorb phosphorus, while sediment from riverbeds and riverbanks did. This suggests that farm fields may not play an important role in mitigating bioavailable phosphorus during high flow events. Broadly, the project’s findings will help improve the effectiveness of phosphorus management in the Maumee River watershed to reduce algal blooms on Lake Erie.
“We’re starting to see these connections between where the sediment comes from, how long it stays in the stream system, and how it’s influencing bioavailable phosphorus,” Hood said. “I think that’s really cool.”
“Where sediment comes from is determined by land use choices, such as whether or not we have cover crops on fields, how we how we treat those fields, how we treat our streams and ditches, and the amount of flow that we have through those rivers,” he continued. “These are choices that we’re making about how to manage this landscape, and that’s having these downstream influences on bioavailable phosphorus.”
Questions about this study? Contact Hood at hood.211@osu.edu. To learn more about other HABRI projects, visit Ohio Sea Grant’s website.
Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.